Ep 141 COVID-19 Part 5 Epidemiology and Prediction Models

Ashleigh Tuite infectious diseases mathematical modeler discusses factors predicting spread of SARS CoV-2, prediction models and flattening the curve of COVID-19. Is the COVID-19 virus likely to mutate? And if it does what are the most likely consequences? What do prediction models show in terms of how long this pandemic will last? Is there likely to be a 2nd peak?…

This podcast and blog post are based on Level C evidence – consensus and expert opinion. Examples of protocols, checklists and algorithms are for educational purposes and require modification for your particular needs as well as approval by your hospital before use in clinical practice.

Key COVID-19 epidemiologic measurements

Based on a systematic review published March 31st, current evidence suggests the following key COVID-19 epidemiologic measurements:

Doubling time: 3-7 days for the epidemic to double in size

Reproduction number – R0 (number of expected secondary cases arising from a single individual, a measure of the transmissibility of a disease): ranges from 1.9 to 6.5, probably between 2.0 and 3.0 (comparable to SARS-CoV, HINI outbreak and a bit more than seasonal influenza which is thought to be about 1.5)

Incubation period: 4-6 days (similar to SARS Co-V and MERS)

Serial interval: 4-8 days

Case fatality rate: ranged from 0.3% to 1.4% (outside China)

Duration of viral shedding: highly variable ranging from 8 to 37 days (viral RNA levels may be higher early after symptom onset)

Is SARS Co-V2 likely to mutate and become more virulent? Is there immunity post-infection?

When SARS C0-V RNA virus replicates it is very error prone, so it does mutate, with estimated mutation rate of 1-2 per month (slower than influenza) however most mutations will be neutral and will not effect how the virus behaves. The virus will likely be relatively stable over the next few months and is unlikely to mutate into a more virulent and deadly virus, and we will likely be able to develop an effective vaccine within 15-18 months. Weak evidence suggests that some of the antibodies that are induced in those with COVID-19 are protective but it is not known whether all infected patients mount a protective immune response and how long a protective effect might last.

According to the CDC it is unclear if there is immunity post-infection.

Which preventative measures help the most to curb transmission of SARS-CoV2?

One of the drivers of the Reproduction number – R0 is the number of contacts people in the region each have per day. A combined strategy of workplace distancing, quarantine of exposed individuals (and less so) school closures and airport screening, seem to be highly effective at containing spread, however the effectiveness of these control measures could be curtailed by a significant portion of asymptomatic patients (17.9%), and pre-symptomatic transmission (12.1%).

Responsive social distancing plan

According to University of Toronto mathematical prediction models, the worst case scenario (limited measures to curb transmission and minimal testing), 50-70% of the population of Ontario would contract the virus and the number of ICU beds required would be 20 times the current maximum capacity. Strict social distancing would need to be in place for 6-12 months to stop ongoing transmission. However, if we adapt a flexible plan whereby we employ social distancing for a few weeks initially, followed by responsive social distancing or staggered return to work/school (with the trigger for social distancing being ICU bed capacity), the mathematical model would predict that the rate of illness would be low enough that we would safely be able to return to work/school (at least partially).

A study of the COVID-outbreak in China showed that social distancing measures were most effective if the staggered return to work occured at 3-4 months after the outbreak started. Apparently staggered return to working at this interval reduced the median number of infections by more than 92%.

Is there likely to be a 2nd peak? Is COVID-19 likely to be a seasonal illness?

Whether on not summer will attenuate the spread of CoV-2 in the northern hemisphere is unknown. If it is attenuated in warmer weather there may be a 2nd peak in the fall similar to SARS CoV-1. A second peak may also occur if social distancing measures that have been lifted are not implemented soon enough after the case rate starts to increase.

Greenfieldboyce, N. Do You Get Immunity After Recovering From A Case Of Coronavirus? March 20, 2020. https://www.npr.org/sections/goatsandsoda/2020/03/20/819038431/do-you-get-immunity-after-recovering-from-a-case-of-coronavirus. Accessed March 28, 2020.

Dr. Anton Helman is an Emergency Physician at North York General in Toronto. He is an Assistant Professor at the University of Toronto, Division of Emergency Medicine and the Education Innovation Lead at the Schwartz-Reisman Emergency Medicine Instititute. He is the founder, editor-in-chief and host of Emergency Medicine Cases.

One Comment

Thank you Anton for the comments you made before the start of the interview of Part 5. I am getting ready for my overnight shift in the ED in Silver Spring, Maryland (Suburb of Washington DC). I appreciate your candor, as I find myself randomly tearing up from stress, insomnia, fear for myself, family and patients.

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